For many years, production vehicles have been almost exclusively constructed with a relatively lightweight skin of stamped sheet metal or formed fiberglass connected to an underlying main structural frame. Typically, the frame is fabricated of two spaced apart, longitudinally extending box beams connected together at intervals by a series of cross members.
While such a construction provides the necessary rigidity to form the vehicle frame to resist twisting and bending under heavy braking, cornering and accelerating loads, as well as for accommodating towing of other vehicles or the like, it also inherently includes a number of drawbacks.
More specifically, in such a design, the engine and/or other components are necessarily mounted overlying the cross members connecting the longitudinal beams. This results in a relatively high center of gravity, as well as a high hood line which reduces driver visibility. Especially in certain utility vehicles, such as tow tractors of the type used in airports, the higher profile restricts clearances around and under aircraft, and in other tight places. In more conventional vehicles, the high hood line also degrades the aerodynamics of the vehicle. Because of this, the overall operating efficiency of any type of vehicle is reduced when the separate vehicle frame is required.
In an attempt to satisfactorily address these and other problems, vehicles of unibody construction have been developed. An example of an early unibody vehicle may be found in the U.S. Pat. No. 2,637,592 to Karlby. In this unibody construction, the relatively thin sheet metal is formed into a plurality of shell-like sections that are stacked together to form the vehicle. The reinforcement provided by the mating shells, serve to mount the mechanical components of the vehicle, support the weight of the vehicle and serve as well to provide the outer skin. In this way, it is proposed that relatively heavy, separate frame members, including the longitudinal beams and cross members may be eliminated. By eliminating these bulky frame members from the construction, the engine theoretically may be placed lower within the vehicle. Thus, the overall hood line of the vehicle may also be lowered. This concept serves to improve driver visibility and maneuverability, as well as the overall aerodynamics of the vehicle. Further, by eliminating all or most of the relatively heavy frame members from the vehicle, the overall weight of the vehicle is theoretically reduced. As a result, the operating efficiency of the vehicle can be said to be increased.
Still, it should be appreciated that further improvement in vehicle design over this unibody construction and the prior art is possible. More particularly, the stacked shells of these previous unibody designs can be recognized as occupying a significant amount of space. This is especially important in the front of the vehicle since like the longitudinal beams on framed vehicles, the body members unduly restrict or limit access to the engine and other components for routine service and repair work. The clearance for the tires may also be limited. As such, suspension travel must by necessity be shortened to the general detriment of the overall quality of the ride. Likewise, the restriction in clearance limits the sharpness of the turns of the vehicle. The only solution to these problems is to raise the vehicle and provide a wider front track, respectively. These design changes, of course, go against the other desirable criteria recognized by those skilled in the art.
In addition to the above shortcomings, it will also be appreciated that the use of a number of overlapping shells is a relatively expensive way to produce a vehicle. Typically, the shells are stamped from numerous sheet metal pieces that must be held in complicated jigs for assembly and welding. Each of the weld points that is enclosed between the shells is subject to substantial stress as the vehicle is placed under loading. Also, moisture is likely to enter and become trapped in between the shells. This leads to rust and corrosion problems, much like appear in many vehicles in the industry today. A need, therefore, is clearly identified for an improved unified frame/body assembly eliminating the stacked shell concept in favor of a simplified and more economical design.